An Etymological Dictionary of the German Language/blecken
blecken, vb. ‘to show one's teeth, grin,’ from MidHG. blęcken, ‘to become visible, show,’ OHG. blęcchen (Goth. *blakjan). Factitive of a Goth. *blikan, which, according to the law of the permutation of consonants, is cognate with Gr. φλέγω, ‘to burn, shine’ (comp. φλογ- in φλόξ, ‘flame’), Lat. flagro, ‘to burn,’ and the Sans. root bhrâj), ‘to shine.’ OHG. blęcchen also means ‘to lighten, gleam, shine forth,’ For further details see Blitz.